Where can I get the Roland D-20 guide book for MIDI? I looked at RolandUs.com but they didn't have it there.
Have a read of this short Introduction to MIDI PDF from Roland. This will give you an overview of what MIDI is and what it can be used for. Then you might be in a better position to tell us what topics you are interested in exploring further.
The manual actually says:
When using the D-20 in Multi Timbral mode, please read the supplied "MIDI Guide book" before this owners manual.
Apparently the keyboard originally came with the MIDI Guide book. I was unable to find it online.
From what I have seen in the D-50 owners manuals about MIDI and from what I can infer from the D-20 manual, I don't think it would be much help to you.
The document recommended by Mark Heath would be a great place to start. On page 7 it describes that "Multitimbral" simply means that the device can play multiple sounds at the same time.
The original "MIDI Guide Book" supplied with the D-20 and other new Roland synths/samplers can be downloaded from the archive.org website